There has been a recent trend toward reducing the power requirements of portable computers. In order to lower power consumption, much of the integrated circuitry used in personal computers is being redesigned to function at lower voltage levels. Some of the circuitry used in portable computers is being designed to operate at low voltage levels, such as 5 volts and 3.3 volts. This helps to reduce the power needs of such computers.
Flash electrically erasable programmable read only memory (flash EEPROM memory) has recently been used to store the basic input/output startup (BIOS) processes for personal computers, and in particular for portable personal computers. However, erasing and programming the cells of a flash EEPROM memory array requires very accurate voltages. The voltages provided by charge pumps and other circuitry implemented as part of integrated circuits, including voltage reference circuits, typically vary due to factors such as operating temperatures, supply voltages, process variations, and load currents. Moreover, when charge pumps are used to supply the voltage, the voltage level provided at the output terminal of a charge pump tends to vary substantially from a desired voltage value.
A problem encountered when using charge pumps derives from the typical circuit topology, which consists of a series of one or more charge pump cells, each cell having an output including a series diode and a storage capacitance. In such a topology, each charge pump cell transfers charge to its storage capacitance only when its series diode is forward-biased. This leads to significant output voltage ripple when load currents are high relative to charge pump storage capacitance. A large ripple voltage may exacerbate power supply noise within an IC, such as a flash EEPROM, by producing pulsating currents. The inductance of the IC bond wires and lead frames may convert pulsating currents into voltage spikes within the IC. The resulting voltage variation within the IC may be sufficient to cause erroneous internal switching of devices and device misoperation. It is therefore desirable to smooth the output voltage that charge pumps provide.